As it is known in the art, a steam turbine is a device which converts thermal energy of pressurized steam to mechanical energy.
The mechanical energy obtained by a steam turbine may be used for driving a rotor of an electric generator for the production of electrical energy. Particularly, the rotor of the electric generator is driven by means of a turbine shaft that interconnects the above mentioned rotor with the steam turbine.
Commonly, the electric generator is coupled with an alternating current electrical grid (hereinbelow called electrical grid) for distributing the produced electrical energy to the consumers through a plurality of transmission lines. Particularly, in order to obtain a delivery of electrical energy from the electric generator to the electrical grid, it is important that the electric generator and the electrical grid are synchronized such that the frequency of the electric generator matches the frequency of the electrical grid.
However, a grid short circuit in one or more of the transmission lines may occur. In order to clear the grid short circuit, the transmission line at which the latter has occurred is isolated by means of a circuit breaker. The above mentioned event is known as load rejection and results to a drop of electric power at the output of the electric generator. Furthermore, the drop of electric power at the output of the electric generator results to an unbalance between the electrical torque and the mechanical torque of the electric generator. Particularly, the value of the electrical torque of the electric generator becomes smaller than the value of the mechanical torque of the electric generator resulting to an acceleration of the steam turbine. As a result of this acceleration, the frequency of the electric generator becomes higher than the frequency of the electrical grid such that a loss of synchronization between the electric generator and the electrical grid may occur.
In order to prevent such a loss of synchronization, a steam turbine governing system may be used. This system is adapted to maintain the speed of the steam turbine at a speed (known as synchronization speed) wherein the frequency of the electric generator matches the frequency of the electrical grid in order to prevent a loss of synchronization between the electric generator and the electrical grid. Particularly, after the occurrence of a grid short circuit, the steam turbine accelerates and its speed exceeds the synchronization speed such that the frequency of the electric generator becomes higher than the frequency of the electrical grid. The steam turbine governing system serves in regulating the steam turbine speed until the latter returns to the synchronization speed at which the frequency of the electric generator matches the frequency of the electrical grid.
The known steam turbine governing systems comprise a governor for regulating the speed of a steam turbine by regulating the steam flow in the latter. The regulation of the steam flow in the steam turbine is achieved by an arrangement of valves whose operation is initiated on demand of the governor. Particularly, the arrangement of valves is disposed at one or more steam pipes through which the steam is provided by a steam generator to the steam turbine. In order to maintain the above mentioned synchronization after the occurrence of a grid short circuit, the arrangement of valves is activated on demand of the governor in order to limit the speed of the steam turbine when the latter exceeds the synchronization speed.
In one well known type of steam turbine governing systems, the governor regulates the speed of the steam turbine in response to the measurement of the speed of the steam turbine after the occurrence of a grid short circuit. The above mentioned measurement is achieved by a speed sensor being disposed at the turbine shaft. The speed sensor communicates with the governor in order to transfer a speed signal to the latter when the speed of the steam turbine exceeds a value of between 100% and 130% of the synchronization speed. In response to this speed signal, the governor initiates operation of the arrangement of valves which lasts until the speed of the steam turbine becomes equal to the synchronization speed.
In another well known type of steam turbine governing systems, the governor regulates the speed of the steam turbine in response to the measurement of the electric power drop which takes place in case of a load rejection at the output of the electric generator after the occurrence of a grid short circuit. The measurement of the electric power drop may be achieved by means of an electric power sensor being disposed at the output of the electric generator. The electric power sensor communicates with the governor in order to transfer an electric power drop signal to the latter. In response to this electric power drop signal, the governor initiates operation of the arrangement of valves which lasts until the speed of the steam turbine becomes equal to the synchronization speed.
However, the initiation of operation of the arrangement of valves of the above mentioned steam turbine governing systems presents a substantial delay after the occurrence of a grid short circuit. This substantial delay may lead to a loss of synchronization between the electric generator and the electrical grid.
Accordingly, there is a need of improving the current steam turbine governing systems of the prior art in order to ensure the maintenance of the synchronization between the electric generator and the electrical grid after the occurrence of a grid short circuit.